A nozzle body for an injection molding machine for processing plastic materials and other plastifiable materials, such as pulverulent or ceramic materials, is known from DE 15 54 934 A, wherein a piston shaft is moved hydraulically, which from a starting position first drives into the bore of a mouth ring and hence closes the nozzle opening and then serves for generating the holding pressure. Insofar this piston shaft is at the same time closure element and pressure element. Therein a change in volume already occurs when the piston shaft is transferred into closure position.
From PATENT ABSTRACTS OF JAPAN, vol. 014, no. 428 (M-1025), 14. September 1990 & JP 02 169219 A a nozzle body is known comprising a cyclically usable closure and pressure element that is axially movable in the nozzle body to influence the holding pressure which significantly influences the quality of the moldings. In actuating the pressure element, however, a change in volume occurs, what is influencing the process control and hence at least also the cycle by cycle reproducible quality of the moldings.
A nozzle body having a closure element which can be used cyclically, is also known from GB 976,369 PS. The plastified material is injected alternately into two mould cavities, the closure element releasing the runner to the one mould cavity when it closes the other. One pressure cylinder unit respectively is associated with the runners so that, as soon as the closure element closes the one runner, the pressure cylinder unit on the associated mould cavity can influence the holding pressure, whilst material is already injected again into the other mould cavity. The time advantage obtained therewith is based however on the fact that two mould cavities are operated alternately so that this device cannot be applied to a compact nozzle unit which possibly operates only one mould cavity.
It is known furthermore to use a shut-off nozzle on a plastic material injection molding machine depending on the material, said shut-off nozzle opening and closing the nozzle opening cyclically. Closure nozzles of this type are known for example from EP 0 322 497 A2. Closure is effected only at the end of the holding pressure phase which is required for obtaining a qualitatively high-grade molded article without sinks or shrinkholes. However, precisely in the case of thick-walled molded articles a long holding pressure time is required, which in this respect contributes directly to increasing the cycle time.